Define and explain the anatomic, physiologic, and cellular aspects of innate immunity
Innate immunity is basically your body's first line of defense. It acts quickly and it acts against a wide variety of things, but one thing it can't do is annihilate specific targets, as that's the job of the adaptive immune system. The innate immune system can, however, aid in activating the adaptive immune system.
The anatomic aspects of innate immunity are pretty easy to visualise- they mainly consist of barriers that physically prevent the pathogen from getting in. It'd be a lot easier for bacteria to infect our tissues if we weren't covered with skin, or didn't have something to line our respiratory and GI tracts. Many of these barriers are lined with mucus, which serves as an extra layer of protection.
Mucus (made up of glycoproteins called mucins) is secreted by goblet cells, which are specialised epithelial cells. This is not, however, the only thing that epithelial cells secrete. They also constitutively (i.e. constantly) secrete lysozymes and defensins, which also help fight off invaders. Lysozymes, which are also secreted in tears and saliva as well as by phagocytes, are enzymes that cleave the peptidoglycans making up the bacterial cell wall. They are more effective against gram-positive bacteria which have their peptidoglycan layer on the outside. Defensins are amphipathic molecules that slot into the bacterial membrane, forming pores which cause liquid to rush in and the bacteria to lyse.
If a pathogen gets through all of this, an inflammatory response takes place. Inflammation is a process in which chemical mediators cause vasodilation and increased vascular permeability, allowing other immune cells to migrate into tissues and do their job. Inflammation is characterised by four main signs: redness, swelling, heat and pain. The redness and heat come from the increased blood flow, the swelling comes from the increased fluid escaping the vessels (due to increased permeability), and the pain comes from swollen tissue pressing on nerves.
Now to talk about the main cells of innate immunity! I've already mentioned the main immune cells in an earlier post, but here's a quick refresher on the cells most important to innate immunity. Neutrophils are pretty much the first ones on the scene, and they help phagocytose (eat) things. Monocytes, which differentiate into macrophages, are even more efficient at eating stuff and they're also pretty good at secreting cytokines (messengers of the immune system). Natural Killer cells are not phagocytic, but they can secrete cytotoxic granules which cause lysis of cells that have been infected with a virus or whatever.
One question that you might have is how the immune system recognises what to kill and what not to kill. Immune system cells, such as macrophages and neutrophils, have a collection of cell surface receptors that can recognise and bind microbes. Sometimes it helps if the bacteria or whatever is first opsonised- that is, coated with other proteins such as antibodies or complement. Once a phagocytic cell figures out what to kill, it begins eating- the bacteria are attached to bits of the membrane that stick out a bit (pseudopodia), and are then ingested to form a phagosome which fuses with a lysosome where the bacteria is ultimately eaten. Finally, digestion products are released from the cell in the process of exocytosis.
Other important proteins to know about include complement, mannose-binding lectin, C-reactive proteins and coagulation factors. Complement, which we'll go into detail later on in the course, has a whole load of roles from killing microbes, to opsonising them, to activating leukocytes. Mannose-binding lectin also helps in opsonisation, as well as in activation of complement. C-reactive proteins, which are secreted by hepatocytes, do pretty much the same. Finally, coagulation factors can help wall off infected tissue.
Describe the nature of the adaptive immune response
In contrast to the innate immune response, the adaptive immune response takes longer to kick in but it can provide a targeted, specific response against particular antigens. By "particular antigens," however, I don't just mean a small handful: the B- and T-cells of our adaptive immune system can in fact respond to a very large number of pathogens. The adaptive immune response is also responsible for memory- that is, our body's ability to "remember" what it's been attacked by in the past and be able to respond more strongly to those pathogens if they appear again.
One thing that's important to note is that the innate and adaptive immune responses are not completely separate, but instead work together to eliminate antigens. For example, some innate immune cells can secrete cytokines when activated, which in turn leads to activation of other cells of the immune system, including adaptive immune cells. Also, antigen-presenting cells, such as dendritic cells, can bring antigens from the infection site and "present" them to T-cells. This allows T-cells to differentiate into the right warriors to do the job.
Compare and contrast innate vs adaptive immunity
I feel like I've already done this throughout the post, but here's a quick summary:
| Innate | Adaptive | |
| Time scale | Immediate | Longer time to act |
| Receptors | Already encoded in the DNA | Requires rearrangement of DNA |
| Specificity | Non-specific | Specific |
| Memory capacity | No memory | Memory |
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